The car bomb was targeting a rebel checkpoint northeast of Idlib city as reported by Syrian Observatory for Human Rights,.

"Eight fighters from Islamist and other rebel brigades died after a huge bombing carried out by ISIS fighters," the Observatory said.

Clashes erupted almost two weeks ago between coalitions of Islamist and moderate rebels. Both groups are fighting about President Bashar Assad's regime.

The Islamic Front is among those battling ISIS. The Islamic Front is a coalition of Islamist battalions, which issued ISIS an ultimatum late Monday night.

"We are setting ISIS a deadline of no more than 24 hours to stop its abuses, free the prisoners and hand over its weapons," the statement said.

Should ISIS fail to accept the warning, the Front said it would "deal with ISIS's presence and actions the same way as with the Assad regime."

In the past, the jihadist group has ignored similar deadlines put in place by rebel groups.

The clashes continued Tuesday, the Observatory said, including fighting in Raqqa, a stronghold of ISIS. It is also known to be the only regional capital to fall from regime hands.

The World Health Organization and UN Children's Fund UNICEF expressed concerns after the fighting caused an interruption in a polio vaccination campaign which began after 17 cases of the disease were reported in Syria last year.

Since March 2011, more than 130,000 people have been killed in the Syrian conflict.